Shape Memory Effect and Pseudo-elasticity
Materials which undergo martensite transformation may exhibit "Shape Memory Effect" and "Pseudo-elasticity." During the transformation on cooling, the high temperature phase known as"austenite" changes its crystalline structure through a diffusionless shear process adopting a less symmetrical structure called "martensite", and, on heating, the reverse transformation occurs. The starting temperature of the cooling transformation is referred to as the M.sub.s temperature and the finishing temperature, M.sub.f. The starting and finishing temperatures of the reverse transformation on heating are referred to as A.sub.s and A.sub.f respectively.
Materials exhibiting Shape Memory Effect can be deformed in their martensitic phase and upon heating recover their original shapes. These materials can also be deformed in their austenitic phase above the A.sub.f temperature through stress-induced martensitic transformation and recover their original shapes upon unloading. This strain recovery, referred to as"pseudo-elasticity" [sometimes referred to herein as "PE"] is associated with the reversion of stress-induced martensite back to austenite. A well known shape memory alloy is nitinol, a near-stoichiometric alloy of nickel and titanium.